Semiconductor devices typically include a semiconductor wafer section such as a semiconductor die having a circuit (i.e. front) side with circuitry thereon, and a noncircuit (i.e. back) side. To protect the semiconductor die, the die can be encapsulated in a plastic resin material or protected by a thin passivation layer.
A semiconductor device can further include one or more conductor-filled openings which extend from the circuit side to the noncircuit side of the semiconductor die, referred to as through-substrate vias (TSV's) or through-silicon vias. TSV's are vertical electrical connections that extend from one of the electrically conductive levels formed on the top surface of a wafer or IC die (e.g., contact level or one of the metal interconnect levels) to the backside (bottom) surface. As a result, a TSV device can be bonded face-up and utilize vertical electrical paths to couple to other IC devices (e.g., on a die, wafer) or to mount to a receiving substrate. The vertical electrical paths are significantly shortened relative to conventional wire bonding technology, generally leading to significantly faster device operation.
To fabricate a TSV wafer including a plurality of dice, openings can be formed within one or more dice on the wafer to a depth less than the full wafer thickness using chemical etching, laser drilling, or one of several energetic methods, such as reactive ion etching (RIE). Once the vias are formed, a dielectric liner can be formed in the opening to provide electrical isolation from the surrounding substrate, then the opening is filled with a conductor (e.g., copper, tungsten, or doped polysilicon) to form embedded TSV's. The bottom of the embedded TSV is generally referred to as an embedded TSV tip. Since most electrically conductive filler materials are metals that can degrade minority carrier lifetimes (e.g., copper or tungsten), a barrier layer is generally deposited on the dielectric liner. In the case of an electroplated metal (e.g., copper) process, a seed layer is generally added after the barrier layer.
A back grinding step can be used to thin the wafer by removing a sufficient thickness of the substrate from the bottom surface of the wafer to reach the embedded TSV tip to expose the electrically conductive filler material at the distal end of the TSV tip. The high substrate removal rate provided by the back grinding process is needed for manufacturability of the thinning process due to the large substrate thickness being removed. A subsequent polish step can be used to remove a thickness of material from the bottom surface of the substrate in an attempt to reduce the mechanical damage and contamination generated by the back grinding process. Alternatively or additionally, a wet or dry chemical etch can be used to reduce the mechanical damage and the contamination resulting from the back grinding.
In one process, the distal end of the completed TSV tip is flush with the bottom surface of the substrate. In another process, a silicon etch is performed such that the TSV's protrude from the back of the wafer, then a protective layer is formed over the back side of the TSV wafer prior to singulating (i.e. singularizing) the plurality of dice. The protective layer can include a nonconductive film (NCF) formed over the back of the wafer to a depth which exposes the distal end of the TSV.
After forming the TSV die, it can be mounted with the circuit side facing away from the receiving substrate in a “face-up” position. After attachment to the receiving substrate, electrical connections such as solder balls can be applied to the TSV's at the front side of the die, and another device can be connected to the TSV die with the solder balls.
Various processes can be performed to protect the electrical connection between the TSV die and the receiving substrate. In one process, conductive structures such as solder balls are applied to either the TSV's at the back side of the die or to landing pads on the receiving substrate, then a nonconductive paste (NCP) is applied to the receiving substrate. The TSV die is placed into the paste with the TSV's aligned with the landing pads, and the solder balls are connected on local reflow using a thermal compression bonding or mass reflow to effect electrical connection between the TSV die and receiving substrate. The nonconductive paste is then cured such that the solder connections are protected.
In another process, referred to as “capillary unclean” (CUF), the TSV die is electrically connected to landing pads of the receiving substrate, and a nonconductive liquid encapsulation is placed at one or more edges of the TSV die. Due to capillarity resulting from the close proximity of the TSV die to the receiving substrate, the liquid encapsulation is drawn between the TSV die and receiving substrate, where it is cured to provide protection.
Once attachment to the receiving substrate is made, TSV's can be used to transfer a signal from the circuit side of the TSV die to the back side, for example to provide back-side access to a ground node on the front of the die. TSV's can also be used to pass a signal through the die, for example from another device mounted to the front of the TSV die, to the receiving substrate.
In another process, the receiving substrate is a semiconductor wafer, which is diced after the attachment of a plurality of TSV dice to provide a plurality of semiconductor devices, each device including two wafer sections. Each of the plurality of semiconductor devices can be attached to a PCB or other receiving substrate.